This invention relates to a process for the production of coated abrasives using a novel dual-curing binder system.
In the conventional production of coated abrasives, a backing material is coated with a first resin coat, known as a maker coat, and a layer of abrasive particles are deposited thereon either by gravity coating or by an electrostatic projection, ("UP"), process. The function of the maker coat is to act as a primary anchor firmly bonding the grits to the backing. This maker coat is cured to ensure that the bond is firm before the main coating that holds the grits rigidly during grinding is applied. This is known as the size coat. The size coat is then cured, and occasionally a supersize coat is applied over the top to provide a grinding aid, anti-static additive or other adjuvant close to the point at which the coated abrasive contacts the surface to be ground when in use.
For many years phenolic resins have been the preferred component of the size coat on account of their excellent physical properties. They have also been preferred as the maker coat, partly because of their excellent adhesion to conventional backing materials and phenolic size coats. By using such similar binder coats it is possible to partially cure the maker and complete the cure at the same time as the cure of the size coat. Phenolics are also popular because they are cheap and because they are applied in an aqueous solution such that no organic solvents that need to be recycled or disposed of in an environmentally acceptable manner are involved.
Phenolic resins have drawbacks however, including the need to remove water before cure is initiated. In addition the prolonged heating required to complete a uniform cure without blistering often lasts many hours. The process of curing is usually operated in a continuous mode wherein a coated abrasive sheet many meters in length is fed slowly into long ovens. The ovens in which the cure occurs are called festoon ovens and the product to be cured is draped in long folds over support slats and these folds move at a pre-determined rate through the oven. The supports over which the sheet is folded often cause defects on the back of the sheet and a misorientation of the grain in the other surface where the maker resin is receiving the initial cure.
For this reason there have been many suggestions for replacement of phenolic resins by other binder products. It has been proposed for example, to use acrylate resins, urea-formaldehyde resins, polyurethane resins, polyester resins, melamine resins, epoxy resins, and alkyd resins.
Some of these are curable by radiation treatment such as by the use of UV light or electron beam radiation. These can be quite expensive and have limitations on the amount of conventional filler material because the particles can prevent effective cure of the parts of the resin binder in the "shadows" behind the particles where little or no radiation penetrates. UV cure radiation has a quite shallow depth of cure in most situations in fact. Electron beam radiation has greater depth of cure but if the dosage is large, the backing material may be deteriorated, leading to premature product failure.
The other binders proposed, while often being well-adapted to specialized uses such as lightweight or waterproof abrasives or very fine grit abrasive products, in general do not provide sufficient strength and efficiency to displace the versatile phenolic resins that are used in the greatest number of coated abrasive products.
A binder formulation has now been discovered that is extremely versatile and effective, particularly when used as a maker coat and the present invention provides a process for making coated abrasive using such a binder.